The keywords meta tag seems to get a lot of attention, but the description meta tag (meta content="This is the description" name="description" /) is one that actually matters.
Not for search engine optimization in the sense of helping a page appear higher in the search results.
No, the description meta tag is more a search engine marketing tag, and it should be written strictly for humans, not for search engine robots. If your page has a good relevant description meta tag, and if the keywords in the description meta tag are a good match for the keywords in the search, there is a high likelihood that your description is what will appear as the description, or snippet, underneath the page title on the search engine results page (SERP).
If that description is well-written (from a marketing perspective) it may well be what convinces the user to click through to that page, rather than to one of the other pages appearing on that SERP.
I see WAAAAAY too many websites that have no description tag, or that use the same description on every page. Every page on a site should include a custom, unique description of that specific page.